Hi -
I think this might be a little simpler:
void test_add_long1(long * result, long * a, long * b, long size) {
__m128i *A = (__m128i*)a;
__m128i *B = (__m128i*)b;
__m128i *end = A + size/2;
__m128i *R = (__m128i*)result;
for(; A < end; ++A, ++B, ++R) {
*R = _mm_add_epi64(*A, *B);
}
}
though I believe if you let the the compiler know about the alignment
of result, a, and b, it will properly optimize this:
void test_add_long2(long * result, long * a, long * b, long size) {
long *end = a + size;
for(; a < end; ++a, ++b, ++result) {
*result = *a + *b;
}
}
Brian
On Thu, Mar 13, 2008 at 5:12 PM, JP Fournier <jape41@xxxxxxxxx> wrote:
> Brian Dessent wrote:
> >
> > You're violates the C aliasing rules. You can't store through a casted
> > pointer like that. You also don't have to do the load/store, the
> > compiler know what you want when you use a union instead:
> >
> > union { __m128i v; long l[2]; } a, b, c;
> >
> > a.l[0] = a.l[1] = 1;
> > b.l[0] = b.l[1] = 1;
> >
> > c.v = _mm_add_epi8 (a.v, b.v);
> > printf("c0=%ld c1=%ld\n", c.l[0], c.l[1]);
>
> Many Thanks Brian. My little program now behaves better:
>
> bash-3.1$ gcc -O2 -msse2 -o sse2 sse2-1.c
>
> bash-3.1$ ./sse2
> c0=2 c1=2
>
>
> #include <stdio.h>
> #include <emmintrin.h>
>
> void test_int() {
>
> union { __m128i v; long l[2]; } a, b, c;
>
> a.l[0] = a.l[1] = 1;
> b.l[0] = b.l[1] = 1;
> c.l[0] = c.l[1] = 0;
>
> c.v = _mm_add_epi8( a.v, b.v );
> printf("c0=%ld c1=%ld\n", c.l[0], c.l[1] );
> }
>
> int main( int count, char ** args ) {
> test_int();
> return 0;
> }
>
>
> >
>
> > There's an even more natural way to do this though using gcc's built-in
> > vector extensions without any of the Intel mmintrin.h stuff. This way
> > will result in code that will vectorize to altivec, sse2, spu, whatever
> > the machine supports, it's not hardware specific:
> >
> > typedef int v4si __attribute__ ((vector_size (16)));
> >
> > v4si a = { 1, 2, 3, 4 }, b = { 5, 6, 7, 8 }, c;
> >
> > c = a + b;
> >
> > You can use all the normal C operators like + and * as if they were
> > scalars but they will be compiled using the corresponding SIMD
> > instructions. See
> > <http://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html> for more. If
> > you want access to the individual parts you can again use the union,
>
> My thinking is that I'd like try to be compiler independent, so by using
> the intel intrinsics I figure I should be able to get gcc and the intel
> compiler to work as a start.
>
> What I am _really_ trying to do is to implement is the addition of
> elements of two arrays.
>
> Is there a more efficient way of doing this than this way?:
>
>
>
> #include <stdio.h>
> #include <emmintrin.h>
>
> void test_add_long(long * result, long * a, long * b, long size) {
> union { __m128i v; long l[2]; } temp1, temp2, temp3;
> int index=0;
>
> for( index=0; index < size; index+=2 ) {
> temp1.l[0] = a[index];
> temp1.l[1] = a[index+1];
> temp2.l[0] = b[index];
> temp2.l[1] = b[index + 1];
>
> temp3.v = _mm_add_epi8( temp1.v, temp2.v );
> result[index] = temp3.l[0];
> result[index+1] = temp3.l[1];
>
> printf("c0=%ld c1=%ld\n", result[index], result[index+1] );
>
> }
> }
>
> int main( int count, char ** args ) {
> // array of 4 8 byte ints
> long a[] = { 1, 2, 3, 4};
> long b[] = { 1, 2, 3, 4};
> long result[] = {0,0,0,0};
>
> test_add_long(result, a, b, 4);
>
> return 0;
> }
>
>
>
>
>
>
>
